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A long time has passed since I moved up from Laser dinghies and Hobie Cats to my first little cruising boat, a Hurley 22, with crazed hatches, faded teak and, joy of joys, a smoky old inboard diesel. I polished and cleaned and fixed the sails, but I don’t think I worried much about engine alignment. Yet now it seems to be often on my mind. Truth is, there’s a big difference between a one pot Yanmar driving a three-quarter-inch shaft and an 85hp engine turning one twice the size.

Engine misalignment can do serious damage from leaking stern glands and transmission failures, to noise and rattles and increased wear and tear.

So let’s take a look at a typical installation and see what that entails. By typical I mean a standard cruising boat installation with an inboard engine and a traditional shafting arrangement with a stuffing box or a mechanical seal and an external cutlass bearing in a strut. And this is where we must start, with the boat out of the water.

The shaft must be straight and this can only be checked by a machine shop. At the same time have them check that the coupling will fit square and true. I’ve found brand new couplings to be over 0.005 inch out. The next step is to fit the shaft to the boat. Sounds obvious but there are things to check that are often overlooked. The shaft must pass through the strut in such a manner that it is absolutely in line with the axis of the cutlass bearing whilst at the same time passing through the centre of the stern tube. Slide the shaft through the strut to a point just before it enters the stern tube. Push the shaft up and down and side to side; the centre point of this movement should align with the centre of the stern tube. If it does then all is well. If not you’ll need to check the condition of the strut itself. That’s beyond the scope of this article. Now slide the shaft into the boat and move inside.

Before we fit the shaft seal it’s a good idea to do a ‘dry run’, but first a word about engine mountings. Big ships have thrust bearings, huge great things, but most cruising yachts rely on the engine mounts to take the strain. Good quality, correctly installed mounts will last for years. Cheap, stressed or dirty ones won’t! A good check before you haul is to go full ahead from stationary whilst watching the mounts and the shaft where it exits the seal. Does it move forward? Now go astern, does the engine leap back on its mounts? One-eighth of an inch is probably okay, much more really isn’t.

Now that we’re happy with the engine mounts let’s go back to the shaft. Without support it will be resting on the bottom edge of the stern tube. Fit the coupling and slide the shaft until the register locates in the recess in the gearbox part of the coupling. Check now that the shaft is in the centre of the stern tube. If not, the engine and gearbox needs lifting or lowering or moving from side to side as needed. Adjust all four mountings until it’s in line.

Next check that the faces of the coupling are parallel. With the two halves held firmly together, but not bolted, check all the way around with your feeler gauges. The difference in the gaps at any given point should be no more than 0.001 inch per inch of coupling diameter. And for couplings over six-inches you should aim for better, say a maximum of 0.0005 per inch.

Adjustment is fairly straightforward once you get the hang of it. The mounts are bolted down through slotted holes and can be loosened off and gently levered from side to side. This will adjust the side to side gaps. Only move the front mounts at this stage; pushing the engine to port will close coupling on that side, and vice versa. On my engine about 1mm (0.040 inch) movement gives about 0.001 inch at the coupling, but of course each configuration will be different. Once that’s done do the top and bottom adjustment using the jacking nuts on each of the front mounts.

Now you’re getting close to your target, check again the shaft is still central in the stern tube. It won’t be! You’ll probably need to do this process of linear and then angular alignment three or four times to achieve the alignment you need. Once you get there, remove the coupling, fit the shaft seal, refit the coupling and check everything again. Double check and then refit the prop, repaint the bottom and finish the 1001 other jobs you hauled out to do.

On launch day, get those feeler gauges out again. It’s vital to recheck alignment once the boat is afloat and fully loaded. Your reward will be a trouble-free drive train and a smoother ride.

Sim Hoggarth is a British merchant navy marine engineer now cruising in the Caribbean with his wife Rosie on board their yacht Wandering Star.